metamorphic
Gneiss
Gneiss (High-grade metamorphic rock)
Hardness: 6-7 on Mohs scale; Color: Tan, brown, and grey with distinct color banding; Luster: Dull to pearly; Crystal structure: Coarse-grained foliated texture; Specific Gravity: 2.6-2.9.
- Hardness
- 6-7 on Mohs scale
- Color
- Tan, brown, and grey with distinct color banding
- Luster
- Dull to pearly
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Tan, brown, and grey with distinct color banding; Luster: Dull to pearly; Crystal structure: Coarse-grained foliated texture; Specific Gravity: 2.6-2.9.
Formation & geological history
Formed by high-pressure and high-temperature regional metamorphism of pre-existing igneous or sedimentary rocks. The banded appearance (gneissic foliation) results from the separation of minerals into light and dark layers during intense pressure.
Uses & applications
Used in construction as crushed stone, dimension stone for flooring and facades, and as a decorative landscaping rock.
Geological facts
Gneiss is some of the oldest rock found on Earth, with some specimens estimated to be over 4 billion years old. It is the result of extreme metamorphic conditions that stop just short of melting the rock into magma.
Field identification & locations
Identify by the characteristic 'gneissic banding' where minerals are segregated into distinct light and dark stripes. Found in the cores of mountain ranges and ancient continental shields.
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